The invention relates to a method for the injection molding of objects of at least two independent parts, which are connected nondetachably but mutually rotatably the subsequently produced part being injected at least partially into a mold recess of the previously produced part or injected at least partially about the previously produced part.
There are a plurality of utensils, which are produced by injection molding from different plastics. Among these utensils, there are also those, which consist of several movable parts, which are connected with one another. The parts involved can be connected by a joint, rotate about an axis, carry out linear motions or also, in some cases, carry out very complex motions. Previously, such parts were made individually and assembled or allocated to one another in expensive manual or automatic operating processes. It has meanwhile become known that, starting out from a part that is produced first, the parts, which are to be connected with this nondetachably but movably, can be molded directly onto the first part. For this purpose, the part produced first forms a cutout of the mold for the part or parts formed subsequently. Fusion of the parts to one another is prevented owing to the fact that a plastic, which can be exposed briefly to high temperatures, is used for the part produced first.
In order to obtain a nondetachable, yet movable connection, an undercut must be molded to the plastic part produced first. Since such an undercut for a detachable, but, at the same time movable connection of two parts in almost all cases must be spherical, an at least 2-part mold, the two halves of which abut in the region of the undercut, is required to produce such an undercut. This is so because the removal of the injection molding, which forms a cutout during the production of a subsequent plastic part in the mold of that part, can take place only owing to the fact that the first mold is separated in the region of the undercut. However, the use of a mold, which is to be separated in the region of the undercut, is associated with a plurality of disadvantages. There will already be an edge in the injection molding if the two halves of the mold are not aligned with the utmost precision and have only a minor mismatch. Since especially this region, which has the undercut, forms a cutout of the mold in the subsequent step of the operation for the next injection molding, a surface unevenness, complementary to the edge of the first part, is formed when this next injection molding is produced. These mutually engaging edges form, as it were, a stop for the relative motion of the two connected parts. Moreover, these surface unevennesses result in increased frictional forces. Not all the difficulties are eliminated even if the two halves of the mold are aligned extremely accurately. If there is the slightest damage to the edge of one or both mold halves, a burr is formed at the part formed first. This burr gives rise to a complementary groove on the injection molding produced subsequently, so that a locking element consisting of a locking nose and a corresponding depression is formed. This also contributes to irregular rotational movements. Since the rotational mobility is limited appreciably or even made impossible by burrs larger than a particular size, relatively short service lives are observed for the molds in order to avoid rejects. Appreciable expenses arise out of this for the production of molds.